The Theory of Plate Tectonics

Vocabulary

Plates: The lithosphere is broken into separate sections called plates.
Scientific: Theory: A well-tested concept that explains wide range of observations.
Plate Tectonics: The geological theory that states that pieces of Earth's lithosphere are in constant, slow motion,
driven by convection currents in the mantle.
Faults: Breaks in Earth's crust where rocks have slipped past each other-form along these boundaries.
Transform boundary: A place where two plates slip past each other, moving in opposite directions.
Divergent boundary: The place where two plates move apart, or diverge.
Rift valley: A deep valley that forms along the divergent boundary.
convergent boundary: The plates where two plates come together, or converge.

outline

A Theory of Plate Motion

How can Earth's plates move?
- Answer: The plates of the lithosphere float on top of the astenoshpere. Convection currents rise in the asthenosphere and spread out beneath the lithosphere.
No plate can budge without affecting the order plates surrounding it.

Plate boundaries

The edges of different pieces of the lithosphere-Earth's rigid shell-meet at lines called plate boundaries.
There are 3 kinda of plate boundaries: transform boundaries, divergent boundaries, and convergent boundaries.
- For each type of boundary, there is a different type of plate movement.

Transform Boundaries

Along transform boundaries crust is neither created nor destroyed.
- Earthquakes occur frequently along these boundaries.

Divergent Boundaries

Most divergent boundaries occur at the mid-ocean ridge.
- Divergent boundaries also occur on land.
When a divergent boundary forms on land, two of Earth's plates slide apart.
A deep vally called a rift valley forms along the divergent
- Example: the Great Rift Valley in east Africa marks a deep crack in the African continent that runs for about 3,000 kilometers.
The rift may someday split the eastern part of Africa away from the rest on the continent.
As a rift valley widens, its floor drops, and eventually the floor may drop enough for the sea to fill the widening gap.

Convergent Boundaries

When two plates converge, the result is called a collision
- Collisions may bring together oceanic crust and oceanic crust, oceanic crust and continental crust, or continental crust and continental crust.
When two plates collide, the density of the plates determines which one comes out on top.
Oceanic crust which is made mostly out of basalt is more dense than continental crust, which is made mostly out of granite,
- Oceanic crust becomes cooler and denser as it spreads away from the mid-ocean ridge
Sometimes a plate carrying the oceanic crust collides with a plate carrying continental crust. The less dense continental crust can't sink under the more dense ocean crust
- Instead, the oceanic plate begins to sin and plunges beneath the continental plate.
Both continental plates are mostly low-density the mantle.
- Instead, the plates crash head-on.
The collision squeezes the crust into mighty mountain ranges

The Continents' Slow Dance

The plates amazingly slow rates: from about one to ten centimeters per year!
The North American and the Eurasian plates are floating apart at a rate of 1.5 centimeters per year
This doesn't seem much, but these plates have been moving for tens of millions of years.
About 260 million years ago, the continents were joined together forming a super continent called pangaea.
- Then about 225 million years ago, Pangaea began to break apart.

ELAJ 1-5

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